[BUG] Newly created LLM nodes include deprecated "prompt" field and section in YAML

[epamLDadayan]

Title: Newly created LLM nodes include deprecated "prompt" field in YAML

Description:
When creating a new LLM node in a pipeline, the generated YAML includes a deprecated prompt field. This field has been replaced by system and task in input_mapping and should not appear in new nodes.

Steps to reproduce:

1. Create a new pipeline or open an existing one.
2. Add a new LLM node.
3. Switch to YAML view and inspect the LLM node structure.

Actual Result:
The YAML includes:

nodes:
  - id: LLM 1
    type: llm
    prompt:
      type: string
      value: ''
    input_mapping:
      system:
        type: fixed
        value: ''
      task:
        type: fixed
        value: ''
      ...

Expected Result:
The prompt field should not be present in newly created LLM nodes. Only input_mapping with system and task should be included:

nodes:
  - id: LLM 1
    type: llm
    input_mapping:
      system:
        type: fixed
        value: ''
      task:
        type: fixed
        value: ''
      chat_history:
        type: fixed
        value: []
    ...

Notes (optional):

• The prompt field is deprecated; remove it from the node creation template.
• Existing nodes with prompt may need migration logic, but new nodes should not include it.

[HawkQing1]

@epamLDadayan it should be fixed on dev, please verify it.

[epamLDadayan]

Verified on DEV env